Bananas are one of the oldest fruits in the world, and they are also fruits and important cash crops with special ties to China. It is a popular "happy fruit" because it contains tryptophan, which can be converted to serotonin, and a "super fruit" favored by fitness enthusiasts because it is rich in minerals such as potassium and sodium.
A man carries bananas at a market in Lahore, Pakistan, on October 16 Xinhua/AFP
But few people know that the evolutionary history of bananas for thousands of years is also a long history of struggle with microbes. It has been on the verge of extinction many times and has been given a new lease of life under the protection of scientists. Recently, reports such as "Blight Disease Ravages and Bananas Are on the Verge of "Extinction" published by the "Nikkei Shimbun" have once again triggered discussions about the survival of bananas.
To what extent can blight affect banana "longevity"? Is there any sign of a reduction in the production of this fruit, which produces more than 100 million tonnes a year in the world? In response to these problems, the reporter interviewed a number of scientific researchers in the fields of tropical fruits, microorganisms, gene editing, etc., tracking the competition between science and technology and microorganisms behind this widely loved fruit.
Text| Zhang Manzi
This article is reproduced from the WeChat public account "Global Magazine" (ID: GlobeMagazine), originally published on November 18, 2022, with the original title "The Power of Technology Behind the Evolution of Bananas".
1 The "disappearance" of the big mic
Articles such as "Blight Disease Ravages Bananas on the Verge of "Extinction" write that banana wilt bacteria that harm bananas are raging around the world, and bananas may face extinction. Biological experts say that the claim that a fruit will "go extinct" may be alarmist, but the harm caused by pathogenic bacteria to bananas is not unfounded.
Under the erosion of the pathogen that causes banana wilt, the commercial variety "Big Mack Banana" that has swept the global market is on the verge of extinction.
Before bananas were "domesticated" by humans, wild bananas were once considered to be a "unreliable fruit" with teeth, astringent taste and difficult to feed because of the hard black seed in the flesh, as well as the special growth environment and short ripening period. It was not until the 19th century that the value of bananas was discovered.
At the beginning of the 19th century, the French naturalist Nicolas Baudin was traveling in Southeast Asia and accidentally found that there was a banana that was different from wild bananas: its flesh was seedless, and it tasted particularly sweet and soft, so he brought this "variant" of wild bananas back to the botanical garden and tried to grow it.
On August 14, 2019, Wu Qijun, head of the healthy secondary seedling breeding base for bananas and pink bananas without blight in Xingye County, Guangxi, inspected the growth of seedlings Photo by Cao Yiming/This magazine
The "seedless banana" cultivated into a live banana with thin skin, thick flesh, rich fruit aroma and sweet taste quickly attracted the attention of botanists and the love of fruit farmers, and it quickly spread throughout many places in a short period of time. Around 1870, Baker, the captain of the American "Telegraph", brought seedless bananas from Jamaica to the United States and founded the Boston Fruit Company, which began to import and promote this high-quality banana variety in large quantities. For a time, bananas became one of the hottest fruits in the United States at that time, and were affectionately known as "Big Mike".
However, Big Mac also welcomed its natural enemy, Fusarium oxysporium. The tenacious fungus, called Fusarium acusporum Cuban transformative, spreads rapidly through the soil where bananas grow and destroys the banana's vascular bundle (which transports water and nutrients for the banana), causing the banana to eventually wither and die like a patient who cannot eat.
According to biological experts, this fungus can lurk in the soil and can survive for more than 50 years, and it widely survives on running water, irrigation water, rainwater, agricultural tools and seedlings, with strong adaptability and transmission.
"As soon as the fungus touches the root system of a banana tree, the entire plantation quickly becomes infected with Panama disease (also known as Fusarium banana wilt), and the entire land is abandoned and forced to switch to other crops, which is very costly." Li Chaosheng, director of the banana genetic improvement research office of the Institute of Biology of the Guangxi Academy of Agricultural Sciences, said.
Fusarium oxysporium Cuban transformed No. 1 physiological species (FOC1) was first discovered in Australia in 1874, and a pandemic in Panama, Central and South America, in 1910, led to the destruction of a large number of banana plantations. The pathogenic bacteria spread outward mainly in Panama, and experts named the disease "Panama disease". The disease continued to spread in the then well-known banana-producing areas, causing huge losses. By the 50s of the last century, it was difficult for Big Mac to carry out large-scale cultivation and production, and could only withdraw from the international trade market.
Replacing Big Mack for extensive cultivation and trade around the world is a budding plantain that is naturally resistant to FOC1. However, in the 60s of the last century, the Cuban transformed physiological subspecies No. 4 (FOC4), which can make Fiancia disease, began to appear. The new pathogen, although most of its genetic sequences are similar to FOC1, is more powerful than FOC1 — it can infect almost all banana varieties.
According to experts, FOC4 infected the Philippines in the 70s and spread to Indonesia and Malaysia in the 90s, and in the following decades, the pathogen was found in major banana-producing regions around the world. After FoC4 was first detected in Taiwan in 1967, the same pathogen was discovered in Guangdong in 1996. The pathogen spread to Fujian in 2000, Hainan in 2001, Guangxi in 2006 and Yunnan in 2007. Up to now, the world's well-known banana producing areas have suffered from it to varying degrees and suffered losses.
"Guangxi's banana planting area was more than 1.8 million mu at its peak, but due to the harm of blight and other reasons, it plummeted to more than 600,000 mu at the lowest point, reducing the area by about two-thirds, and the banana area in Guangdong, Hainan, Yunnan and other places has also been greatly reduced." In a sense, Li said, blight is equivalent to "cancer" in bananas.
2 There are so many "banana killers"
Why can a fungus be devastating to a fruit?
According to banana breeding experts, the bananas we eat now are triploids obtained by humans after a long period of variety improvement. Compared to their diploid "ancestor" wild bananas, triploid bananas have no seeds and taste better, but they are highly sterile and can only rely on asexual reproduction to reproduce. Asexual reproduction has made the genomes of some commercially grown bananas completely identical, so there is no difference in disease resistance. This crop, which lacks genetic diversity, can endanger entire populations if infected with a fungus or virus.
The characteristics of banana plants themselves also make it difficult to effectively eliminate Fusarium cepticonus transmitted through soil.
"Banana is a perennial plant, a good banana orchard can last for 8~12 years. The root system of bananas is very developed, so the prevention and control of soil-borne diseases is very difficult, and even if there is a highly effective agent, it is difficult to apply it to every surrounding of the root system. Sun Xianyun, an expert at the Institute of Microbiology of the Chinese Academy of Sciences who has studied fungi for many years, explained, "Bananas cannot be uprooted or replanted after harvesting in a short period of time, like vegetables, because it is perennial and the upfront investment is particularly large." For those orchards that have been planted for 3~5 years and are at their peak fruiting, once soil-borne diseases occur, the losses will be very heavy. ”
In addition to banana wilt, which threatens the safety of the banana industry worldwide, bananas are also infested with diseases such as black stripe leaf spot, black star disease, gray streak disease, anthrax, and bundle top disease. According to a list of banana diseases published by the Institute of Environment and Plant Protection of the Chinese Academy of Tropical Agricultural Sciences, bananas suffer from 74 different types of diseases including fungal diseases, bacterial diseases, virus and phytoplasma diseases, nematode diseases, physiological diseases, etc., and banana wilt is the main disease, and the harm is increasingly serious.
A vendor sells bananas at a traditional village market in Dhaka, Bangladesh, September 16, 2020 Xinhua/Ouxin
3 Biotechnology starts banana "defense"
According to incomplete statistics, about 120 countries and regions around the world produce bananas. Meanwhile, bananas are the fourth largest food crop in the world and one of the staple foods for nearly 600 million people.
Humans have solved the problem of the global spread of blight FOC1 by finding alternative disease-resistant varieties of sprouts. But the germs and viruses that are deadly to bananas are also evolving and mutating, refusing to hibernate easily, trying to survive between changes in plant varieties and advances in human technology.
"There is a co-evolution between plants and microbes, which is a natural phenomenon." Li Huaping, director of the Plant Protection Function Research Office of the National Banana Industry Technology System and professor at the College of Plant Protection of South China Agricultural University, said that any kind of microorganism will undergo adaptive mutations in order to survive.
Li Huaping introduced that the blight FOC4, which currently threatens the world banana industry, originated from FOC1, which was the "culprit" that once caused Big Mac to be on the verge of extinction. When the foc1-resistant plantain became the main planting variety, the foc1 that was not infected with the plantain originally underwent adaptive mutation and acquired the ability to infect the plantain.
"Fusarium oxysporum is a large population, there are some genome differences in the population, and in the process of gene exchange between species, a population suddenly acquires the ability to infect the plantain, resulting in the infection of the plantain. It is now tentatively speculated that the main gene source of FOC4 is FOC1, and some gene sources are other populations of Fusarium oxyspora. "This is a long-term, adaptive co-evolution process." ”
FOC4, mutated from FOC1, is more powerful than the former: it can infect almost all banana varieties and is almost "invincible". But unlike 70 years ago, when Big Mac fell into extinction, people are no longer helpless in the face of banana wilt.
Until the forties and fifties of the last century, people still had relatively little research on the occurrence of this disease, and the technical means of prevention and control were not advanced enough, so the big Mack banana was on the verge of extinction. "Our country began to set up a banana industry technology system in 2008, taking this disease as a key problem to overcome, and in more than ten years, we have established a relatively complete comprehensive prevention and control system." Li Huaping said.
This comprehensive prevention and control technology system has its own focus on the use of disease-resistant varieties, seedling quarantine, soil conditioning, biocontrol agent application, agricultural operation, water and fertilizer management and other whole-process management measures for banana wilt no-disease, mild-disease areas and severely diseased areas, which can prevent and reduce the occurrence and harm of banana wilt.
Li Huaping introduced that for the newly reclaimed disease-free banana areas, measures such as planting soilless substrate seedlings, establishing a separate water intake system or water body disinfection, and disinfecting vehicles and personnel entering the garden are promoted to prevent the introduction of banana wilt bacteria; For mild disease areas with an incidence of less than 30%, a comprehensive prevention and control technology system is established with soil pathogen detection as the guide, soil improvement as the basis, disease-resistant varieties as the core, and beneficial biocontrol fertilizers as supplements, so as to avoid tillage or cultivate less tillage as the supporting comprehensive prevention and control technology system. For banana orchards with an incidence of more than 40%, soil disinfection is carried out to reduce the content of pathogenic bacteria in the soil, and other crops are planted for more than 3 years before bananas are replanted using intercropping, crop rotation, and set seeding.
Every link in the comprehensive prevention and control system needs to be closely coordinated, and the cultivation of disease-resistant varieties is the top priority. Due to the fact that cultivated bananas are mostly triploid, as well as the lack of seeds and high sterility, which makes conventional cross-breeding of bananas very difficult, scientists around the world are trying to breed disease-resistant varieties through various technical means.
In China, through bud breeding, radiation mutagenesis and other breeding methods, researchers have cultivated several disease-resistant varieties with good resistance and excellent biological traits, including "Nantian Yellow", "Baodao Banana", "Gui Banana No. 9", "Zhong Heat No. 1" and "Zhong Heat No. 2". Medium-to-blight resistant varieties such as "Nantian Yellow" and "Baodao Banana", which have been widely promoted and planted in China, are not yet 100% immune to blight, but can resist a certain concentration of pathogens in the soil.
A more cutting-edge genetic breeding tool is gene editing technology. Australia's Queensland University of Technology and the Centre for Horticultural Innovation spent $9.8 million using CRISPR gene-editing tools to implant disease-resistant genes into banana genomes. The research team of the Institute of Pomology, Guangdong Academy of Agricultural Sciences, China, also established this set of gene editing technology applied to bananas in 2017, laying the foundation for further using gene editing technology to apply genetic improvement in bananas.
However, there is still some way to go from establishing gene-editing technology to breeding disease-resistant varieties for large-scale production.
"The editing efficiency of gene editing technology is very high, which is a great improvement over traditional breeding methods." Sun Xianyun said, "At present, only the technology of CRISPR gene editing has been established in bananas, which can speed up the pace of breeding, but there have been no reports of breeding banana varieties with good disease resistance and particularly good taste." ”
"Gene editing technology is of course a very forward-looking new direction, but at present it is only in the laboratory stage and has not yet been applied to production." Li Chaosheng said.
At present, the market access conditions for gene-edited crops are not clear, and it is unknown whether gene-edited foods can be accepted by the market.
"What is certain is that pathogenic bacteria and plants have long been in the co-evolution process of the 'arms race'. Plants cannot escape the infection of pathogenic bacteria, so they have evolved a strategy of co-living with pathogenic bacteria. This is the smart way for plants to survive. From the story of the evolution of bananas, we also need to realize how important it is to protect the diversity of banana species and varieties. Sun Xianyun said. (Intern Lui Yao also contributed to this article)
Uncle Ku benefits
Uncle Ku's book giveaway is always on! Chinese Minmin University Press provided Uncle Ku with 20 copies of "Jiangshan and the People: An Analysis of China's Governance System" to enthusiastic readers. This book elevates the governance experience of traditional China and the Communist Party of China to philosophy, sociology and psychology, answers how the Party and the government face the differences between the people, and tries to use new ideas to explain what is the "governance system" of contemporary China—its historical evolution and innovation, its advantages and characteristics, what universal significance and Chinese characteristics it has, and how it personally connects everyone. Please comment under the article, and the top 3 with the highest likes (more than 30) will receive a free book.